The Fischer-Tropsch process was developed in the 1920's as a way of producing hydrocarbons from synthesis gas, i.e., hydrogen and carbon monoxide. Initially, the process was centered on producing gasoline range hydrocarbons as automotive fuels. Today, however, the Fischer-Tropsch process is increasingly viewed as a method for preparing heavier hydrocarbons such as diesel fuels, and more preferably waxy molecules, for conversion to clean, efficient lubricants. Indeed, the importance of producing a product slate containing a higher carbon number distribution is ever increasing. A measure of the carbon number distribution is the Schulz-Flory alpha value, which represents the probability of making the next higher carbon number compound from a given carbon number compound. The Schulz-Flory distribution is expressed mathematically by the Schulz-Flory equation:Wi=(1−α)2iαi−1 where i represents carbon number, α is the Schulz-Flory distribution factor which represents the ratio of the rate of chain propagation to the rate of chain propagation plus the rate of chain termination, and Wi represents the weight fraction of product of carbon number i. Alpha numbers above about 0.9 are, in general, representation of wax producing processes, and the higher the alpha number, e.g., as it approaches 1.0, the more selective the process is for producing wax molecules.
The waxy Fischer-Tropsch products, of course, have poor cold flow properties limiting their value unless converted into more useable products. Thus, the Fischer-Tropsch wax is subjected to treatments such as hydrotreating, hydroisomerization and hydrocracking to convert the wax to more valuable material. Hydroisomerization is particularly preferred treatment method for converting the wax to a more valuable material. Indeed, heavy lube basestocks are separated from the hydroisomerized material by high temperature distillation.
The practical usefulness of high temperature distillation in separating a slate of heavy lube base stocks is somewhat limited. Typically, high temperature distillation units are suitable for conducting distillation at temperatures up to about 1050° F. (566° C.) equivalent atmospheric boiling point. Commercial wiped-film evaporative distillation units can be used to raise the effective boiling range but are costly for large volume applications. Thus, there remains a need for an effective method for fractionating heavy lube molecules from isomerized Fischer-Tropsch wax.
Accordingly, an object of the present invention is to produce heavy lube base stocks from Fischer-Tropsch wax.
Another object of the invention is to provide a method for separating hydroisomerized Fischer-Tropsch wax into high viscosity fractions suitable as lube base stocks.
Other objects of the invention will become apparent from that herein which follows.